It has heretofore been known to applying a liquid composition on to a recording medium and applying thereto light including active energy rays to cure a curable substance in the liquid composition, thereby forming a cured film to form an image. For example, a liquid composition (curable emulsion paint) obtained by emulsifying and dispersing a non-aqueous active energy ray curable substance in an aqueous vehicle and adding a coloring material to this emulsion is applied to a medium, and the liquid composition is then irradiated with ultraviolet rays to form an image.
In recent years, such an active energy ray curable liquid composition and various paints using the liquid composition have been widely applied to graphic art, sign art, production of display panels, label recording, package recording and production of electron circuit boards.
Among these, it has been known to apply such a liquid composition containing the active energy ray curable substance as an ink to an ink jet recording method. When the active energy ray curable liquid composition is used in such an ink jet recording method, it is considered that a non-aqueous or aqueous material is used as a curable substance used therein. When the non-aqueous curable substance is used, the ink is roughly classified into 2 types. One of the types is the so-called oil ink obtained by dispersing a pigment in an organic solvent such as toluene or methyl ethyl ketone. The other of the types is the so-called 100% curable ink (non-solvent ink) containing a liquid monomer or oligomer, and a pigment dispersion without using such an organic solvent as described above.
However, when such an oil ink as described above is used, it is necessary to sufficiently care for environment because the organic solvent is easy to volatilize off in the air. The 100% curable ink is liable to cause irregularities between a recorded portion and a non-recorded portion because all the ink components applied on to a recorded medium have to be converted to a cured film. It is thus difficult to give a glossy feeling to an image. Therefore, it is difficult under the circumstances to use such ink to applications in which high image quality is required.
On the other hand, when the aqueous curable substance is used, an aqueous solvent containing water as a main component is used as a solvent, so that a burden on environment by volatilization of the solvent is relatively small. It is also possible to reduce the occurrence of such irregularities as caused in the 100% curable ink. From such reasons, it is extremely useful to use the active energy ray curable liquid composition using the aqueous curable substance in the ink jet recording method. From such circumstances, there has been a demand for development of aqueous active energy ray curable liquid compositions and, at the same time, development of various hydrophilic reactive resins applicable to the liquid compositions.
Under the circumstances, hydrophilic curable substances having both acidic group and (meth)acryloyl group or vinyl group are known as one example of aqueous active energy ray curable substances. Examples of such materials include esters of succinic anhydride and 2-hydroxyethyl(meth)acrylate, esters of orthophthalic anhydride and 2-hydroxyethyl(meth)acrylate, and vinylnaphthalenesulfonic acid.
Curable substances to which hydrophilicity has been given by a polyethylene oxide chain are known as an example of industrially produced water-soluble compounds having 2 or more polymerizable functional groups in their molecules. Examples of such compounds include (meth)acrylates of polyhydric alcohols, such as diethylene glycol di(meth)acrylate and tetraethylene glycol di(meth)acrylate.
Polyfunctional hydrophilic curable substances are also disclosed (see Japanese Patent Application Laid-Open No. H08-165441). The compounds disclosed therein are used for the purpose of increasing the number of hydroxyl groups in a molecule as a technique for imparting hydrophilicity.
(Meth)acrylates of hydrophilic polyepoxides derived from polyalcohols are also disclosed (see Japanese Patent Application Laid-Open Nos. 2000-117960 and 2002-187918). The compounds disclosed therein can satisfy curability by active energy rays and physical properties of cured materials to some extent, and the viscosities of aqueous solutions of the compounds also satisfy the level required of ink-jet inks.
Further, compositions for energy ray curable powder paints having an ethylenically unsaturated group containing compound in addition to a spiro ring containing (meth)acrylate compound are disclosed (see Japanese Patent Application Laid-Open No. 2003-165927).
However, it has not been sufficient in some cases to use such above-mentioned compounds as described in the various prior art documents as main materials for such active energy ray curable liquid compositions as applicable to the ink jet recording method. In, for example, the compounds having one polymerizable functional group in one molecule, in many cases, a polymerization rate is slow and the crosslinking degree of a cured film is low, because such a compound has only one polymerizable functional group in one molecule. Therefore, a liquid composition using such a compound hardly becomes a main material for the purpose of the present invention because curability and properties of the cured film are poor in view of the requirement of higher durability in recent years.
According to investigation by the present inventors, it has been found that the water-soluble compounds which have 2 or more polymerizable functional groups in one molecule and are industrially produced involve the following problem. More specifically, these compounds are poor in water solubility when the ethylene oxide chain is short. When the ethylene oxide chain is long to the contrary, the water solubility is achieved, but properties of the cured film, particularly hardness, adhesion and stickiness, are not sufficient in some cases.
The compounds described in Japanese Patent Application Laid-Open No. H08-165441 have water solubility and are curable by active energy rays. However, the compounds are used in ink receiving layers for recording media, and whether such a compound can be applied to the object of the present invention or not is unknown. According to an investigation by the present inventors, the compounds involve a problem that when a liquid composition containing such a compound is prepared, its viscosity becomes too high, and so a range of application is limited.
According to investigation by the present inventors, the compounds disclosed in Japanese Patent Application Laid-Open Nos. 2000-117960 and 2002-187918, such as (meth)acrylates of hydrophilic polyepoxides involve the following problem. More specifically, when such an aqueous curable substance having a (meth)acrylate group is used as a material for aqueous inks, the following problems may have occurred in some cases. A dye dissolved in an aqueous medium by an anionic group, or a pigment dispersion obtained by dispersing a pigment in an aqueous medium by an anionic group is widely used as a coloring material for an aqueous ink. On the other hand, when such an aqueous curable substance as described above is contained in the aqueous ink, the pH of the aqueous ink may be lowered to an acid range in some cases attending on the formation of acrylic acid by hydrolysis of the (meth)acrylate group. In such a condition, a problem may be caused from the viewpoint of storage stability of the aqueous ink in some case, because the dye stably existing in the ink in itself at the original pH adjusted to an alkaline or neutral range precipitates or the pigment dispersion aggregates or precipitates. In addition, a problem may be caused from the viewpoint of ejection stability in some cases because the curable substance in the ink forms insoluble matter by an unexpected reaction, which is particularly marked in an ink jet recording method that the ink is ejected by the action of thermal energy.
The compound described in Japanese Patent Application Laid-Open No. 2003-165927 is used in the composition for powder paints. Whether the compound can be applied to a liquid composition or not and whether it is water-soluble or not are unknown. In addition, whether sufficient curability can be achieved or not is also unknown because the publication describes that a monofunctional maleimide compound or the like is preferred.